The known polycrystalline aluminum nitride molded bodies however, have in part less desirable properties than pure aluminum nitride. The properties depend to a great extent on the amount of impurities that are present in the molded body and particularly, the amount of oxygen, carbon and metals present. For example, the theoretical value of the thermal conductivity of pure, monocrystalline aluminum nitride is 320 W/mK, which drops to about 50 W/mK as the oxygen content increases. Still lower values are obtained on further increase of the oxygen content or in two-phase aluminum nitride ceramics (see G. A. Slack in J. Phys. Chem. Solids (1973), Vol. 34, pp. 321-335; ref. in C.A. Vol. 78 (1973), No. 129,310 r).
The strength at high temperatures also depends on the impurities present in the molded body. The bending strength drops sharply at temperature above 1000.degree. C. in comparison to the value measured at room temperature which is ascribed to the presence of oxygen-containing phases at the grain boundaries in the aluminum nitride sintered body.
Pure aluminum nitride does not sinter easily because of its predominantly covalent bonding. In order to obtain high density bodies, it was deemed necessary either to start from aluminum nitride powders rich in oxygen or to add sintering aids preferably metal oxides which aid compression during hot pressing. It is possible to obtain from aluminum nitride powder having an oxygen content of 1.0% by weight, (2.1% by weight based on Al.sub.2 O.sub.3) by axial hot pressing, a molded body of aluminum nitride with 98% theoretical density (herein-after abbreviated as % TD), which at room temperature has a bending strength of 265 N/mm.sup.2 and which at 1400.degree. C. dropped to 125 N/mm.sup.2 (see DE-A-14 71 035 corresponding to U.S.-A-3,108,887).
By hot pressing commerically available aluminum nitride powders at 2000.degree. C., sintering densities of 97 to 99% TD were obtained. The purest of the polycrystalline aluminum nitride molded bodies thus produced contained 0.9% by weight oxygen, had a density of 97% TD and a thermal conductivity of 66 W/mK (see G. A. Slack et al. in Amer. Ceram. Soc. Bull. (1972) Vol. 71, pp. 852 to 856; ref. in C.A. Vol. 78 (1973) No. 19686 K and DE-A-20 35 767).
Polycrystalline aluminum nitride molded bodies produced by hot pressing commercially available aluminum nitride powder without sintering aids at a hot pressing temperature of 1700.degree. C., had 0% porosity, contained 0.8% by weight oxygen and had a bending strength of 375 N/mm.sup.2, measured according to the 3-point method at room temperature, which at 1300.degree. C. dropped to about 225 N/mm.sup.2 (see P. Boch et al. in Ceram, Int. 1982, Vol. 8 (1), pp. 34-40; ref. in C.A. Vol. 97(1982) No. 59846 m).
It has been reported from Japan, in relation to metallic impurities, that it is possible to hot press high-purity aluminum nitride powders without sintering aids at temperatures of 2000.degree. C. to form dense transparent bodies. Physical data were given only for monophase aluminum nitride bodies produced with an admixture of 0.5 percent by weight calcium oxide as a sintering aid and containing from 0.5 to 0.7% by weight oxygen. A hot pressed aluminum nitride body had a density of 99.6% TD, a thermal conductivity of 91 W/mK and a bending strength of 510 N/mm.sup.2 measured at room temperature according to the 3-point method. A pressureless sintered aluminum nitride body had a density of 99.1% TD and a thermal conductivity of 95 W/mK (see N. Kuramoto et al. in J. Mater. Sci. Lett. 1984, Vol. 3 (6), pp. 471-474; ref in C.A. Vol. 101 (1984), No. 42402s).
Aluminum nitride molded bodies produced by conventional hot pressing methods, with biaxial application of pressure, have an anisotropic microstructure so that their properties depend on direction.
Since only bodies of simple shape can be produced by a hot pressing process, pressureless sintering processes have been developed for the production of polycrystalline aluminum nitride molded bodies. Pressureless sintering processes for aluminum nitride require use of sintering aids to obtain high sintered densities. Numerous compounds have been tested for promoting sintering of aluminum nitride. Especially effective are oxides of elements from the 2nd and 3rd group of the Periodic System including the lanthanides (see K. Komeya et al. in Yogyo Kyokaishi; 1981, Vo. 89 (6), pp. 330-336; ref. in C.A. Vol. 95 (1981), No. 155257 z).
Due to the sensitivity of aluminum nitride to impurities, in particular oxygen impurities, it is necessary to use the least possible amounts of oxygen containing sintering aids or to reduce, by additional processing steps, the oxygen present in the aluminum nitride powder and/or the oxygen introduced by the sintering aids.
According to the process disclosed in U.S. Pat. No. 4,435,513, a mixture of commercially available aluminum nitride powders having an oxygen content of not more than 5% by weight, together with up to 5.66% by weight of alkaline earth oxide sintering aids with up to 6.54% by weight carbon in the form, for example, carbon black or a carbonizable organic material such as sugar or phenolic resin were pressureless sintered at temperatures of up to 2000.degree. C. The carbon in the mixture prevents the formation of aluminum oxide nitride phases and the amount of oxygen present in the starting aluminum nitride powder is reduced. As can be seen from the examples, the aluminum nitride molded bodies produced had a density of 98.5% TD and a thermal conductivity of 63 W/mK. By a subsequent hot isostatic pressing treatment, the density could be increased to 99% TD and the thermal conductivity to 71 W/mk.
According to the process disclosed in EP-A-147 101, aluminum nitride powders containing 0.001 to 7% by weight oxygen mixed with 0.01 to 15% by weight oxides of rare earth metals were hot pressed or sintered without pressure. It is believed that the oxygen present in the aluminum nitride starting powder reacts with the oxides of the rare earth metals (preferably Y.sub.2 O.sub.3) forming compounds (phases) having a garnet or Perowskite structure so that oxygen does not diffuse into the aluminum nitride lattice with formation of mixed crystals or aluminum oxy-nitride phases (AlN polytypes). The garnet or Perovskite phases are formed during sintering at relatively low temperatures (1000.degree. to 1300.degree. C.), they melt at high temperatures (1600.degree. to 1950.degree. C.) and induce a liquid-phase sintering that results in dense bodies. As can be seen from the examples, the best results with regard to thermal conductivity were obtained with aluminum nitride bodies prepared from aluminum nitride powders having oxygen contents of 0.3 to 1.0 percent by weight with admixture of 0.1 to 3.0% by weight Y.sub.2 O.sub.3 which were pressureless sintered at 1000.degree. C. For an aluminum nitride (AlN) body with the relatively high oxygen content of approximately 0.9% by weight (0.6% by weight from the AlN powder+about 0.3 by weight oxygen from the 1.5% by weight Y.sub.2 O.sub.3), the highest heat conductivity given in all the examples was 135 W/mK. By X-ray diffraction analysis there were detected in these bodies, together with the main aluminum nitride phase, small amounts of an Al-Y garnet phase and an aluminum oxynitride phase, which are present as oxidic impurities at the aluminum nitride grain boundaries.
According to the process disclosed in EP-A-13 32 75 (corresponding to U.S. Pat. No. 4,478,785 and U.S. Pat. No. 4,533,645), it was disclosed that sintering aids were not necessary and only a carbon-containing material was used. Commercially available aluminum nitride powders of high purity, with regard to metallic impurities, and containing approximately 1.5 to 3.0% by weight oxygen were partially deoxidized by adding carbon so that the aluminum nitride powders or the green body prepared therefrom still contained, after the deoxidation treatment by heating, a residual oxygen content of from about 0.35 to about 1.1% weight. The high residual oxygen content is necessary for pressureless sintering, at temperatures in the range of 1900.degree. to 2200.degree. C., to sintered densities of more than 85% TD in resulting sintered bodies. Accordingly, the finished aluminum nitride sintered bodies have a residual oxygen content in the range of about 0.35 to about 1.1% by weight and a residual carbon content in detectable amounts as low as about 0.2% by weight. The sintered bodies of aluminum nitride prepared by the process are stated to be free of secondary phases which is understood to mean that they contain less than about 1% by volume secondary phases (that is, phases other than AlN). As can be seen from the examples, however, the lack of sintering aids produces sintered bodies with final densities in the range of 91.6 to 97.2% TD and relatively high residual oxygen contents. In addition, despite the use of an aluminum nitride starting powder of high purity in relation to metallic impurities, the thermal conductivity values at room temperature were at a maximum of only 82 W/mK.
According to the process disclosed in EP-A-15 25 45, the improvement in thermal conductivity is obtained, by using for deoxidation of the aluminum nitride, an admixture containing yttrium such as yttrium metal, yttrium hydride and/or yttrium nitride instead of carbon. The yttrium reacts with the oxygen present in the aluminum nitride forming liquid phases containing yttrium and oxygen, which, at the same time act as sintering aids in the pressureless sintering step. After cooling, these phases remain in the aluminum nitride sintered body as secondary phases at the grain boundaries of the aluminum nitride. The composition according to point F in the phase diagram, has the smallest amount of these secondary phases, (and corresponds to 1.6 equivalent percent Y and 3.2 equivalent percent oxygen), which corresponds to 6.2% by weight of a YAlO.sub.3 secondary phase, or, differently expressed, to 1.81% by weight oxygen and 3.36% by weight Y in the AlN sintered body. As it can be seen from the examples, the highest value for thermal conductivity was 174 W/mK for an AlN sintered body containing Y.sub.2 O.sub.3 and Y.sub. 4 Al.sub.2 O.sub.9 as secondary phases.
As can be seen from the extensive prior art, it has not hitherto been possible to produce polycrystalline aluminum nitride bodies having a high density that do not contain substantial amounts of impurities such as oxygen, carbon and/or metals, which unfavorably affect the thermal conductivity by changing the lattice parameters of the aluminum nitride crystals and/or unfavorably affect resistance to high temperatures by inclusion of impurity containing phases at the grain boundaries of the aluminum nitride crystals.